A known electric power source installed in a fuel cell electric vehicle is a hybrid system in which a reformer, a fuel cell unit and a storage battery are installed. A hybrid system of this type is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2000-315511 (JP-A-2000-315511). In a construction disclosed therein, electric power is distributed from the fuel cell unit or the storage battery, via an electric power regulator, to a vehicle-driving traction motor, and accessories of the fuel cell unit, and the reformer, in accordance with driver's operation of an accelerator. The storage battery capacity provided in the vehicle is set at an amount that, when the system is started, allows supply of electric power to the traction motor, the accessories of the fuel cell unit only from the storage battery during a period until the reformer is sufficiently warmed up and is able to stably supply a reformed gas to the fuel cell unit.
In a fuel cell electric vehicle having such a hybrid system, the response delay of the fuel cell unit (a time delay prior to performance of steady electricity generation) at the time of detection of a high load request during an intermittent operation mode becomes a problem. The intermittent operation mode refers to an operation mode in which during a low load condition, for example, during idling or deceleration or the like, the operation of the fuel cell unit is temporarily stopped, and the vehicle is driven only on the basis of the supply of electric power from the storage battery. If the intermittent operation duration (duration of pause of operation) of the fuel cell unit becomes long, there is possibility of deterioration of the I-V characteristic (electric current-to-voltage characteristic) of the fuel cell unit from the I-V characteristic occurring prior to the pause of the fuel cell unit. The I-V characteristic of the fuel cell unit fluctuates all the time depending on the cell temperature, the reaction gas flow, the gas pressure and the humidity, and also considerably fluctuates with fluctuations of the internal resistance of a polymer electrolyte membrane caused by changes in the water content as well as states of battery operation (an excessive state, a steady state, etc.). Once the I-V characteristic deteriorates, a restart of the fuel cell unit in response to a high load request does not immediately achieve a sufficient recovery of the cell voltage; specifically, a time of about 1 to 2 seconds is required for the recovery to a sufficient voltage. Besides the response delay of the fuel cell unit, the response delay of the entire fuel cell system, including the accessories thereof, is unignorable. If the response delay of the fuel cell unit becomes equal to or longer than 200 ms, remarkably degraded drivability results. Therefore, there is a strong demand for development of an improvement technology regarding the response delay of the fuel cell unit.
There is another problem to be solved. If the storage battery capacity provided in a fuel cell electric vehicle is insufficient, it becomes impossible to cope with a high load request. On the other hand, provision of an excessively large storage battery capacity is a waste of resources. Thus, there is a need to review the installation design so that a storage battery that accords with the vehicle characteristics will be selected.